ERK2 is required for efficient terminal differentiation of skeletal myoblasts

Transcriptome sequencing reveals the potential mechanisms of dietary lutein regulation on chicken leg muscle development

Lutein is an antioxidant that can indicate the oxidative status of organisms through its coloration and may be involved in the development process of chicken skeletal muscle. In this study, after feeding Nanhai Yellow Chickens with lutein-containing feed for 21 d, the lutein group significantly increased the muscle fiber diameter and decreased the fiber density in the chicken's leg muscles compared to the control group. To elucidate the potential regulatory mechanisms by which lutein is involved in muscle development, RNA-seq was used to detect changes in gene expression in chicken leg muscle tissue. After data analysis, a total of 249 significantly differentially expressed genes (DEG) were identified, including TGF-β superfamily (MSTN and TGFB1) and nonreceptor tyrosine kinase c-Src (SRC). Results from GO and KEGG analysis showed that the DEGs were enriched in GO terms such as positive regulation of the ERK1/ERK2 cascade and negative regulation of myoblast differentiation, as well as signaling pathways including the Toll-like receptor signaling pathway and the MAPK signaling pathway. These significantly enriched GO terms and pathways are closely related to muscle development, suggesting that lutein may play an important role in the process of chicken muscle development. This study provides insights into the regulatory mechanisms of dietary lutein on chicken muscle development.

Hypocalcemia in combination with hyperphosphatemia impairs muscle cell differentiation in vitro

PURPOSE

Hypoparathyroidism is a rare endocrine disorder characterized by low or absent secretion of parathyroid hormone (PTH), which leads to decreased calcium and increased phosphorus levels in the serum. The diagnosis of hypoparathyroidism is based on the identification of the aforementioned biochemical abnormalities, which may be accompanied by clinical manifestations. Symptoms of hypoparathyroidism, primarily attributed to hypocalcemia, include muscle cramps or spasms, facial, leg, and foot pain, seizures, and tingling in the lips or fingers. The treatment of hypoparathyroidism depends on the severity of symptoms and the underlying pathology. Over the long term, calcium supplements, active vitamin D analogs, and thiazide diuretics may be needed. In fact, in patient cohorts in which optimal disease control still remains elusive, replacement therapy with recombinant parathyroid hormone analogs may be contemplated. Despite the predominantly neuromuscular symptoms of hypoparathyroidism, further effects of parathyroid hormone deficiency at the muscle cell level remain poorly understood. Thus, the aim of our study was to evaluate the effects of hypocalcemia in combination with hyperphosphatemia on muscle cells differentiation in vitro.

METHODS

C2C12 cells, an in vitro model of muscle cells, were differentiated for 2 or 6 days in the presence of hypocalcemia (CaCl2 0.9 mmol/l) and moderate (PO4 1.4 mmol/l) or severe (PO4 2.9 mmol/l) hyperphosphatemia, or combinations of both conditions. Cell differentiation and expression of genes linked to muscle differentiation were evaluated.

RESULTS

The combination of hypocalcemia with hyperphosphatemia induced a significant reduction (50%) in differentiation marker levels, such as MyoD (protein 1 for myoblast determination) and myogenin on the 1st day of differentiation, and MHC (myosin heavy chains) after 6 days of differentiation compared to control. Furthermore, this condition induced a statistically significant reduction of insulin-like growth factor-1 (IGF-1) mRNA expression and inhibition of IGF signaling and decrease in ERK phosphorylation compared to control cells.

CONCLUSIONS

Our results showed that a condition of hypocalcemia with hyperphosphatemia induced an alteration of muscle cell differentiation in vitro. In particular, we observed the reduction of myogenic differentiation markers, IGF-1 signaling pathway, and ERK phosphorylation in differentiated skeletal myoblasts. These data suggest that this altered extracellular condition might contribute to the mechanisms causing persistence of symptoms in patients affected by hypoparathyroidism.

Tanshinone IIA promotes the proliferation and differentiation ability of primary muscle stem cells via MAPK and Akt signaling

Salvia miltiorrhiza Bunge is a widely-used traditional Chinese medicine to treat a variety of diseases including muscle disorders. The underlying pharmacological mechanisms of which active component and how it functions are still unknown. Tanshinone IIA (Tan IIA) is the main active lipophilic compound in Salvia miltiorrhiza Bunge. Muscle stem cells (MuSCs) play a crucial role in maintaining healthy physiological function of skeletal muscle. For the purpose of this study, we investigated the effects of Tan IIA on primary MuSCs as well as mechanism. The EdU staining, cell counts assay and RT-qPCR results of proliferative genes revealed increased proliferation ability of MuSCs after Tan IIA treatment. Immunofluorescent staining of MyHC and RT-qPCR results of myogenic genes found Tan IIA contributed to promoting differentiation of MuSCs. In addition, enrichment analysis of RNA-seq data and Western blot assay results demonstrated activated MAPK and Akt signaling after treatment of Tan IIA during proliferation and differentiation. The above proliferative and differentiative phonotypes could be suppressed by the combination of MAPK inhibitor U0126 and Akt inhibitor Akti 1/2, respectively. Furthermore, HE staining found significantly improved myofiber regeneration of injured muscle after Tan IIA treatment, which also contributed to muscle force and running performance recovery. Thus, Tan IIA could promote proliferation and differentiation ability of MuSCs through activating MAPK and Akt signaling, respectively. These beneficial effects also significantly contributed to muscle regeneration and muscle function recovery after muscle injury.

In-ovo feeding with creatine monohydrate: implications for chicken energy reserves and breast muscle development during the pre-post hatching period

The most dynamic period throughout the lifespan of broiler chickens is the pre-post-hatching period, entailing profound effects on their energy status, survival rate, body weight, and muscle growth. Given the significance of this pivotal period, we evaluated the effect of in-ovo feeding (IOF) with creatine monohydrate on late-term embryos' and hatchlings' energy reserves and post-hatch breast muscle development. The results demonstrate that IOF with creatine elevates the levels of high-energy-value molecules (creatine and glycogen) in the liver, breast muscle and yolk sac tissues 48 h post IOF, on embryonic day 19 (p < 0.03). Despite this evidence, using a novel automated image analysis tool on day 14 post-hatch, we found a significantly higher number of myofibers with lower diameter and area in the IOF creatine group compared to the control and IOF NaCl groups (p < 0.004). Gene expression analysis, at hatch, revealed that IOF creatine group had significantly higher expression levels of myogenin (MYOG) and insulin-like growth factor 1 (IGF1), related to differentiation of myogenic cells (p < 0.01), and lower expression of myogenic differentiation protein 1 (MyoD), related to their proliferation (p < 0.04). These results imply a possible effect of IOF with creatine on breast muscle development through differential expression of genes involved in myogenic proliferation and differentiation. The findings provide valuable insights into the potential of pre-hatch enrichment with creatine in modulating post-hatch muscle growth and development.

Therapeutic Myogenesis Induced by Ultrasound Exposure in a Volumetric Skeletal Muscle Loss Injury Model

BACKGROUND

Low-intensity pulsed ultrasound (LIPUS) irradiation has been shown to induce various responses in different cells. It has been shown that LIPUS activates extracellular signal-regulated kinase 1/2 (ERK1/2) through integrin.

PURPOSE

To study the effects of LIPUS on myogenic regulatory factors and other related myogenesis elements in a volumetric skeletal muscle loss injury model.

STUDY DESIGN

Controlled laboratory study.

METHODS

C57BL/6J mice were subjected to full-thickness muscle defect injury of the quadriceps and treated with direct application of LIPUS 20 min/d or non-LIPUS treatment (control) for 3, 7, and 14 days. LIPUS was also applied to C2C12 cells in culture in the presence of low and high doses of lipopolysaccharides. The expression levels of myogenic regulatory factors and the expression levels of myokine-related and angiogenic-related proteins of the control and LIPUS groups were analyzed.

RESULTS

Muscle volume in the injury site was restored at day 14 with LIPUS treatment. Paired-box protein 7, myogenic factor 5, myogenin, and desmin expressions were significantly different between control and LIPUS groups at days 7 and 14. Myokine and angiogenic cytokine-related factors were significantly increased in the LIPUS group at day 3 and decreased with no significant difference between the groups by day 14. LIPUS induced different responses of myogenic regulatory factors in C2C12 cells with low and high doses of lipopolysaccharides. LIPUS promoted myogenesis through short-lived increase in interleukin-6 and heme oxygenase 1, together with activation of ERK1/2.

CONCLUSION

LIPUS had a constant effect on the variables of tissue damage, from macrotrauma to microtrauma, leading to efficient muscle regeneration.

CLINICAL RELEVANCE

The focus of therapeutic strategies with LIPUS has been not only for microvascular regeneration but also for skeletal muscle and related local tissue recovery from acute or chronic damage.

Activation of Lactate Receptor Positively Regulates Skeletal Muscle Mass in Mice

G protein-coupled receptor 81 (GPR81), a selective receptor for lactate, expresses in skeletal muscle cells, but the physiological role of GPR81 in skeletal muscle has not been fully elucidated. As it has been reported that the lactate administration induces muscle hypertrophy, the stimulation of GPR81 has been suggested to mediate muscle hypertrophy. To clarify the contribution of GPR81 activation in skeletal muscle hypertrophy, in the present study, we investigated the effect of GPR81 agonist administration on skeletal muscle mass in mice. Male C57BL/6J mice were randomly divided into control group and GPR81 agonist-administered group that received oral administration of the specific GPR81 agonist 3-Chloro-5-hydroxybenzoic acid (CHBA). In both fast-twitch plantaris and slow-twitch soleus muscles of mice, the protein expression of GPR81 was observed. Oral administration of CHBA to mice significantly increased absolute muscle weight and muscle weight relative to body weight in the two muscles. Moreover, both absolute and relative muscle protein content in the two muscles were significantly increased by CHBA administration. CHBA administration also significantly upregulated the phosphorylation level of p42/44 extracellular signal-regulated kinase-1/2 (ERK1/2) and p90 ribosomal S6 kinase (p90RSK). These observations suggest that activation of GRP81 stimulates increased the mass of two types of skeletal muscle in mice in vivo. Lactate receptor GPR81 may positively affect skeletal muscle mass through activation of ERK pathway.

The Probiotic Phaeobacter inhibens Provokes Hypertrophic Growth via Activation of the IGF-1/Akt Pathway during the Process of Metamorphosis of Greater Amberjack (Seriola dumerili, Risso 1810)

Metamorphosis entails hormonally regulated morphological and physiological changes requiring high energy levels. Probiotics as feed supplements generate ameliorative effects on host nutrient digestion and absorption. Thereby, the aim of the present research was to investigate the impact of the probiotic Phaeobacter inhibens as a water additive on cellular signaling pathways in the metamorphosis of greater amberjack (Seriola dumerili). Activation of insulin-like growth factor type 1 receptor (IGF-1R), protein kinase B (Akt), mitogen-activated protein kinases (MAPKs) and AMP-activated protein kinase (AMPK), induction of heat shock proteins (Hsps), and programmed cell death were assessed through SDS-Page/immunoblot analysis, while energy metabolism was determined through enzymatic activities. According to the results, greater amberjack reared in P. inhibens-enriched water entered the metamorphic phase with greater body length, while protein synthesis was triggered to facilitate the hypertrophic growth as indicated by IGF-1/Akt activation and AMPK inhibition. Contrarily, MAPKs levels were reduced, whereas variations in Hsps response were evident in the probiotic treatment. Apoptosis and autophagy were mobilized potentially for the structural remodeling processes. Furthermore, the elevated enzymatic activities of intermediary metabolism highlighted the excess energy demands of metamorphosis. Collectively, the present findings demonstrate that P. inhibens may reinforce nutrient utilization, thus leading greater amberjack to an advanced growth and developmental state.

Effects of Atmospheric Ammonia on Skeletal Muscle Growth in Broilers

Ammonia, one of the most polluted gases in poultry houses, has always been an urgent problem to solve. Exposure to ammonia can threaten the respiratory tract, induce inflammation, and decrease growth performance. To date, there are few studies investigating the effects of ammonia on skeletal muscle growth. In this experiment, a total of 144 broilers were randomly divided into two groups, and 0 ppm and 35 ppm atmospheric ammonia were administered in the chambers. The trial lasted for 21 days. The breast muscle, thigh muscle, dressed weight, and serum biochemical indexes were measured. The skeletal muscle fibre morphology was observed using light microscopy, and the expressions of genes associated with skeletal muscle development and myosin heavy chain genes were assessed. After 7 days of ammonia exposure, the broilers' weight in the ammonia group decreased. On the 21st day of the experiment, in the ammonia group, the breast muscle weight, thigh muscle weight, and dressed weight decreased, the blood urea nitrogen content increased, skeletal muscle fibre diameter shortened, the expression of myostatin increased, and the expression of myosin heavy chain-FWM and myosin heavy chain-FRM decreased significantly. This article suggests that 35 ppm atmospheric ammonia seriously affects the skeletal muscle gain rate of broilers, and the myostatin pathway could be a potential regulation of the growth rate of muscle fibre under ammonia exposure.

Advances in understanding the mitogenic, metabolic, and cell death signaling in teleost development: the case of greater amberjack (Seriola dumerili, Risso 1810)

Cell growth and differentiation signals of insulin-like growth factor-1 (IGF-1), a key regulator in embryonic and postnatal development, are mediated through the IGF-1 receptor (IGF-1R), which activates several downstream pathways. The present study aims to address crucial organogenesis and development pathways including Akt, MAPKs, heat shock response, apoptotic and autophagic machinery, and energy metabolism in relation to IGF-1R activation during five developmental stages of reared Seriola dumerili: 1 day prior to hatching fertilized eggs (D-1), hatching day (D0), 3 days post-hatching larvae (D3), 33 (D33) and 46 (D46) days post-hatching juveniles. During both the fertilized eggs stage and larval-to-juvenile transition, IGF-1R/Akt pathway activation may mediate the hypertrophic signaling, while p44/42 MAPK phosphorylation was apparent at S. dumerili post-hatching processes and juvenile organs completion. On the contrary, apoptosis was induced during embryogenesis and autophagy at hatching day indicating a potential involvement in morphogenetic rearrangements and yolk-sac reserves depletion. Larvae morphogenesis was accompanied by a metabolic turnover with increased substantial energy consumption. The findings of the present study demonstrate the developmental stages-specific shift in critical signaling pathways during the ontogeny of reared S. dumerili.

Lactate Activates AMPK Remodeling of the Cellular Metabolic Profile and Promotes the Proliferation and Differentiation of C2C12 Myoblasts

Lactate is a general compound fuel serving as the fulcrum of metabolism, which is produced from glycolysis and shuttles between different cells, tissues and organs. Lactate is usually accumulated abundantly in muscles during exercise. It remains unclear whether lactate plays an important role in the metabolism of muscle cells. In this research, we assessed the effects of lactate on myoblasts and clarified the underlying metabolic mechanisms through NMR-based metabonomic profiling. Lactate treatment promoted the proliferation and differentiation of myoblasts, as indicated by significantly enhanced expression levels of the proteins related to cellular proliferation and differentiation, including p-AKT, p-ERK, MyoD and myogenin. Moreover, lactate treatment profoundly regulated metabolisms in myoblasts by promoting the intake and intracellular utilization of lactate, activating the TCA cycle, and thereby increasing energy production. For the first time, we found that lactate treatment evidently promotes AMPK signaling as reflected by the elevated expression levels of p-AMPK and p-ACC. Our results showed that lactate as a metabolic regulator activates AMPK, remodeling the cellular metabolic profile, and thereby promoting the proliferation and differentiation of myoblasts. This study elucidates molecular mechanisms underlying the effects of lactate on skeletal muscle in vitro and may be of benefit to the exploration of lactate acting as a metabolic regulator.

Beyond Mechanical Tension: A Review of Resistance Exercise-Induced Lactate Responses & Muscle Hypertrophy

The present review aims to explore and discuss recent research relating to the lactate response to resistance training and the potential mechanisms by which lactate may contribute to skeletal muscle hypertrophy or help to prevent muscle atrophy. First, we will discuss foundational information pertaining to lactate including metabolism, measurement, shuttling, and potential (although seemingly elusive) mechanisms for hypertrophy. We will then provide a brief analysis of resistance training protocols and the associated lactate response. Lastly, we will discuss potential shortcomings, resistance training considerations, and future research directions regarding lactate's role as a potential anabolic agent for skeletal muscle hypertrophy.

N-acetyl cysteine prevents arecoline-inhibited C2C12 myoblast differentiation through ERK1/2 phosphorylation

Arecoline is known to induce reactive oxygen species (ROS). Our previous studies showed that arecoline inhibited myogenic differentiation and acetylcholine receptor cluster formation of C2C12 myoblasts. N-acetyl-cysteine (NAC) is a known ROS scavenger. We hypothesize that NAC scavenges the excess ROS caused by arecoline. In this article we examined the effect of NAC on the inhibited myoblast differentiation by arecoline and related mechanisms. We found that NAC less than 2 mM is non-cytotoxic to C2C12 by viability analysis. We further demonstrated that NAC attenuated the decreased number of myotubes and nuclei in each myotube compared to arecoline treatment by H & E staining. We also showed that NAC prevented the decreased expression level of the myogenic markers, myogenin and MYH caused by arecoline, using immunocytochemistry and western blotting. Finally, we found that NAC restored the decreased expression level of p-ERK1/2 by arecoline. In conclusion, our results indicate that NAC attenuates the damage of the arecoline-inhibited C2C12 myoblast differentiation by the activation/phosphorylation of ERK. This is the first report to demonstrate that NAC has beneficial effects on skeletal muscle myogenesis through ERK1/2 upon arecoline treatment. Since defects of skeletal muscle associates with several diseases, NAC can be a potent drug candidate in diseases related to defects in skeletal muscle myogenesis.

Global Quantitative Proteomics Analysis Reveals the Downstream Signaling Networks of Msx1 and Msx2 in Myoblast Differentiation

The msh homeobox 1 (Msx1) and msh homeobox 2 (Msx2) coordinate in myoblast differentiation and also contribute to muscle defects if altered during development. Deciphering the downstream signaling networks of Msx1 and Msx2 in myoblast differentiation will help us to understand the molecular events that contribute to muscle defects. Here, the proteomics characteristics in Msx1- and Msx2-mediated myoblast differentiation was evaluated  using isobaric tags for the relative and absolute quantification labeling technique (iTRAQ). The downstream regulatory proteins of Msx1- and Msx2-mediated differentiation were identified. Bioinformatics analysis revealed that these proteins were primarily associated with xenobiotic metabolism by cytochrome P450, fatty acid degradation, glycolysis/gluconeogenesis, arginine and proline metabolism, and apoptosis. In addition, our data show Acta1 was probably a core of the downstream regulatory networks of Msx1 and Msx2 in myoblast differentiation.

Supplementary Information

The online version contains supplementary material available at 10.1007/s43657-022-00049-y.

MEK-inhibitor-mediated rescue of skeletal myopathy caused by activating Hras mutation in a Costello syndrome mouse model

Costello syndrome (CS) is a congenital disorder caused by heterozygous activating germline HRAS mutations in the canonical Ras/mitogen-activated protein kinase (Ras/MAPK) pathway. CS is one of the RASopathies, a large group of syndromes caused by mutations within various components of the Ras/MAPK pathway. An important part of the phenotype that greatly impacts quality of life is hypotonia. To gain a better understanding of the mechanisms underlying hypotonia in CS, a mouse model with an activating Hras^G12V allele was utilized. We identified a skeletal myopathy that was due, in part, to inhibition of embryonic myogenesis and myofiber formation, resulting in a reduction in myofiber size and number that led to reduced muscle mass and strength. In addition to hyperactivation of the Ras/MAPK and PI3K/AKT pathways, there was a significant reduction in p38 signaling, as well as global transcriptional alterations consistent with the myopathic phenotype. Inhibition of Ras/MAPK pathway signaling using a MEK inhibitor rescued the Hras^G12V myopathy phenotype both in vitro and in vivo, demonstrating that increased MAPK signaling is the main cause of the muscle phenotype in CS.

Summary: A Costello syndrome (CS) mouse model carrying a heterozygous Hras p.G12V mutation was utilized to investigate Ras pathway dysregulation, revealing that increased MAPK signaling is the main cause of the muscle phenotype in CS.

Preliminary evidence of differential expression of myogenic and stress factors in skeletal muscle of older adults with low muscle strength

The age-related loss of muscle strength and mass, or sarcopenia, is a growing concern in the ageing population. Yet, it is not fully understood which molecular mechanisms underlie sarcopenia. Therefore, the present study compared the protein expression profile, such as catabolic, oxidative, stress-related and myogenic pathways, between older adults with preserved (8 ♀ and 5 ♂; 71.5 ±2.6 years) and low muscle strength (6 ♀ and 5 ♂; 78.0±5.0 years). Low muscle strength was defined as chair stand test time >15 seconds and/or handgrip strength <16kg (women) or <27kg (men) according the EWGSOP2 criteria. Catabolic signaling (i.e. FOXO1/3a, MuRF1, MAFbx, LC3b, Atg5, p62) was not differentially expressed between both groups, whereas the mitochondrial marker COX-IV, but not PGC1α and citrate synthase, was lower in the low muscle strength group. Stress factors CHOP and p-ERK1/2 were higher (~1.5-fold) in older adults with low muscle strength. Surprisingly, the inflammatory marker p-p65NF-κB was ~7-fold higher in older adults with preserved muscle strength. Finally, expression of myogenic factors (i.e. Pax7, MyoD, desmin; ~2-fold) was higher in adults with low muscle strength. To conclude, whereas the increased stress factors might reflect the age-related deterioration of tissue homeostasis, e.g. due to misfolded proteins (CHOP), upregulation of myogenic markers in the low strength group might be an attempt to compensate for the gradual loss in muscle quantity and quality. These data might provide valuable insights in the processes that underlie sarcopenia.

Egg components reverse the atrophy induced by injury in skeletal muscles

Skeletal muscle atrophy is the loss of muscle tissue caused by factors such as inactivity, malnutrition, aging, and injury. In this study, we aimed to investigate whether egg components exert inhibitory effects on muscle atrophy. An egg mix solution was orally administered for 10 consecutive days to male C57BL/6J mice injected with cardiotoxin in the tibialis anterior (TA) muscle. The administration of egg mixture significantly decreased the atrogin-1 and MuRF-1 protein levels, key factors in muscle atrophy, as observed by western blotting. Furthermore, we investigated the effects of egg components such as avidin, lecithin, biotin, 3-sn-phosphatidylcholine, and L-α-phosphatidylcholine on dexamethasone (DEX)-treated C2C12 myotubes. Lecithin, biotin, 3-sn-phosphatidylcholine, and L-α-phosphatidylcholine in egg yolk significantly recovered the diameters of C2C12 myotubes decreased upon DEX application. Avidin did not show such reversal. Biotin, 3-sn-phosphatidylcholine, and L-α-phosphatidylcholine also attenuated atrogin-1 protein expression enhanced by DEX. Our findings reveal that egg yolk components could contribute to the reversal of skeletal muscle atrophy induced by muscle injury.

Expression profiles of SLC39A/ZIP7, ZIP8 and ZIP14 in response to exercise-induced skeletal muscle damage

BACKGROUND

Zinc transporters are thought to facilitate the mobilization of zinc (Zn) and the role of Zn as a signaling mediator during cellular events. Little is known about the response of Zn movement and zinc transporters during muscle proliferation and differentiation processes after damage.

METHODS

After rats were subjected to one 90-min session of downhill running to cause muscle damage, the gastrocnemius muscles were harvested to assess the expression of zinc transporters SLC39A/ZIP7, ZIP8, ZIP14 and myogenic regulatory factors at the 0 h, 6 h, 12 h, 1 d, 2 d, 3 d, 1 w and 2 w time points after exercise.

RESULTS

SLC39A/ZIP7, ZIP8 and ZIP14 had translocated to different compartments of the cell following damage, and they exhibited differential expression profiles after eccentric exercise. The results regarding the myogenetic regulators showed that nf-κb was upregulated 2 d after exercise, and STAT3 and Akt1 mRNA levels were mostly expressed 2 w after exercise. The upregulation of phosphatidylinositol 3-kinase, catalytic subunit gamma (pik3cg), erk1 and erk2 mostly occurred at the early stage (6 h or 12 h) after exercise. In addition, we found that zip7, zip8 and zip14 expression was moderately correlated with certain markers of muscle regeneration.

CONCLUSION

The zinc transporters SLC39A/ZIP7, ZIP8 and ZIP14 have differential expression profiles upon eccentric exercise, and they might regulate muscle proliferation or differentiation processes through different cellular pathways after exercise-induced muscle damage.

The ESCRT-0 subcomplex component Hrs/Hgs is a master regulator of myogenesis via modulation of signaling and degradation pathways

Background

Myogenesis is a highly regulated process ending with the formation of myotubes, the precursors of skeletal muscle fibers. Differentiation of myoblasts into myotubes is controlled by myogenic regulatory factors (MRFs) that act as terminal effectors of signaling cascades involved in the temporal and spatial regulation of muscle development. Such signaling cascades converge and are controlled at the level of intracellular trafficking, but the mechanisms by which myogenesis is regulated by the endosomal machinery and trafficking is largely unexplored. The Endosomal Sorting Complex Required for Transport (ESCRT) machinery composed of four complexes ESCRT-0 to ESCRT-III regulates the biogenesis and trafficking of endosomes as well as the associated signaling and degradation pathways. Here, we investigate its role in regulating myogenesis.

Results

We uncovered a new function of the ESCRT-0 hepatocyte growth factor-regulated tyrosine kinase substrate Hrs/Hgs component in the regulation of myogenesis. Hrs depletion strongly impairs the differentiation of murine and human myoblasts. In the C2C12 murine myogenic cell line, inhibition of differentiation was attributed to impaired MRF in the early steps of differentiation. This alteration is associated with an upregulation of the MEK/ERK signaling pathway and a downregulation of the Akt2 signaling both leading to the inhibition of differentiation. The myogenic repressors FOXO1 as well as GSK3β were also found to be both activated when Hrs was absent. Inhibition of the MEK/ERK pathway or of GSK3β by the U0126 or azakenpaullone compounds respectively significantly restores the impaired differentiation observed in Hrs-depleted cells. In addition, functional autophagy that is required for myogenesis was also found to be strongly inhibited.

Conclusions

We show for the first time that Hrs/Hgs is a master regulator that modulates myogenesis at different levels through the control of trafficking, signaling, and degradation pathways.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01091-4.

Effects of omega-7 palmitoleic acids on skeletal muscle differentiation in a hyperglycemic condition

Maternal obesity and diabetes are known to be involved in fetal myogenesis, but the later stages of myogenesis are not well understood. In this study, we investigated the influence of a hyperglycemic environment on L6 skeletal myoblast differentiation and the function of omega-7 palmitoleic acids. Exposure to a high concentration of glucose (25 mM) in high-glucose culture medium (HG) increased the expression of myogenic genes (MyoD, Myogenin, MRF4, Myhc2x, and Myhc2a) and the synthesis of myosin. HG also activated the PI3K/AKT pathway revealed muscle cell differentiation. Furthermore, the levels of reactive oxygen species (ROS) and an inflammatory cytokine (Tnfaip3; tumor necrosis factor alpha-induced protein 3), which are crucial for the growth and differentiation of skeletal muscle, were increased by HG. Palmitoleic acids suppressed the expression levels of myogenic regulatory genes and increased the expression level of a cell proliferation-related gene (Pax3). Trans-palmitoleic acid and eicosapentaenoic acid (TPA and EPA) increased the phosphorylation level of MAPK/ERK1/2 and downregulated ROS generation and Tnfaip3 expression. In contrast, cis-palmitoleic acid inactivated MAPK/ERK1/2, leading to increased ROS generation. In conclusion, a hyperglycemic environment mediated by HG induced excessive muscle differentiation. Palmitoleic acids inhibited myoblast differentiation by downregulating muscle-specific genes. Moreover, trans-palmitoleic acids may have beneficial antioxidant and/or anti-inflammatory effects in cells.

Effects of Mini-Dystrophin on Dystrophin-Deficient, Human Skeletal Muscle-Derived Cells

BACKGROUND

We are developing a novel therapy for Duchenne muscular dystrophy (DMD), involving the transplantation of autologous, skeletal muscle-derived stem cells that have been genetically corrected to express dystrophin. Dystrophin is normally expressed in activated satellite cells and in differentiated muscle fibres. However, in past preclinical validation studies, dystrophin transgenes have generally been driven by constitutive promoters that would be active at every stage of the myogenic differentiation process, including in proliferating muscle stem cells. It is not known whether artificial dystrophin expression would affect the properties of these cells.

AIMS

Our aims are to determine if mini-dystrophin expression affects the proliferation or myogenic differentiation of DMD skeletal muscle-derived cells.

METHODS

Skeletal muscle-derived cells from a DMD patient were transduced with lentivirus coding for mini-dystrophins (R3-R13 spectrin-like repeats (ΔR3R13) or hinge2 to spectrin-like repeats R23 (ΔH2R23)) with EGFP (enhanced green fluorescence protein) fused to the C-terminus, driven by a constitutive promoter, spleen focus-forming virus (SFFV). Transduced cells were purified on the basis of GFP expression. Their proliferation and myogenic differentiation were quantified by ethynyl deoxyuridine (EdU) incorporation and fusion index. Furthermore, dystrophin small interfering ribonucleic acids (siRNAs) were transfected to the cells to reverse the effects of the mini-dystrophin. Finally, a phospho-mitogen-activated protein kinase (MAPK) array assay was performed to investigate signalling pathway changes caused by dystrophin expression.

RESULTS

Cell proliferation was not affected in cells transduced with ΔR3R13, but was significantly increased in cells transduced with ΔH2R23. The fusion index of myotubes derived from both ΔR3R13- and ΔH2R23 -expressing cells was significantly compromised in comparison to myotubes derived from non-transduced cells. Dystrophin siRNA transfection restored the differentiation of ΔH2R23-expressing cells. The Erk1/2- signalling pathway is altered in cells transduced with mini-dystrophin constructs.

CONCLUSIONS

Ectopic expression of dystrophin in cultured human skeletal muscle-derived cells may affect their proliferation and differentiation capacity. Caution should be taken when considering genetic correction of autologous stem cells to express dystrophin driven by a constitutive promoter.

Mechanical Stimulation of Adhesion Receptors Using Light-Responsive Nanoparticle Actuators Enhances Myogenesis

The application of cyclic strain is known to enhance myoblast differentiation and muscle growth in vitro and in vivo. However, current techniques apply strain to full tissues or cell monolayers, making it difficult to evaluate whether mechanical stimulation at the subcellular or single-cell scales would drive myoblast differentiation. Here, we report the use of optomechanical actuator (OMA) particles, comprised of a ∼0.6 μm responsive hydrogel coating a gold nanorod (100 × 20 nm) core, to mechanically stimulate the integrin receptors in myoblasts. When illuminated with near-infrared (NIR) light, OMA nanoparticles rapidly collapse, exerting mechanical forces to cell receptors bound to immobilized particles. Using a pulsed illumination pattern, we applied cyclic integrin forces to C2C12 myoblasts cultured on a monolayer of OMA particles and then measured the cellular response. We found that 20 min of OMA actuation resulted in cellular elongation in the direction of the stimulus and enhancement of nuclear YAP1 accumulation, an effector of ERK phosphorylation. Cellular response was dependent on direct conjugation of RGD peptides to the OMA particles. Repeated OMA mechanical stimulation for 5 days led to enhanced myogenesis as quantified using cell alignment, fusion, and sarcomeric myosin expression in myotubes. OMA-mediated myogenesis was sensitive to the geometry of stimulation but not to MEK1/2 inhibition. Finally, we found that OMA stimulation in regions proximal to the nucleus resulted in localization of the transcription activator YAP-1 to the nucleus, further suggesting the role of YAP1 in mechanotransduction in C2C12 cells. These findings demonstrate OMAs as a novel tool for studying the role of spatially localized forces in influencing myogenesis.

Cardiotoxin-induced skeletal muscle injury elicits profound changes in anabolic and stress signaling, and muscle fiber type composition

To improve muscle healing upon injury, it is of importance to understand the interplay of key signaling pathways during muscle regeneration. To study this, mice were injected with cardiotoxin (CTX) or PBS in the Tibialis Anterior muscle and were sacrificed 2, 5 and 12 days upon injection. The time points represent different phases of the regeneration process, i.e. destruction, repair and remodeling, respectively. Two days upon CTX-injection, p-mTORC1 signaling and stress markers such as BiP and p-ERK1/2 were upregulated. Phospho-ERK1/2 and p-mTORC1 peaked at d5, while BiP expression decreased towards PBS levels. Phospho-FOXO decreased 2 and 5 days following CTX-injection, indicative of an increase in catabolic signaling. Furthermore, CTX-injection induced a shift in the fiber type composition, characterized by an initial loss in type IIa fibers at d2 and at d5. At d5, new type IIb fibers appeared, whereas type IIa fibers were recovered at d12. To conclude, CTX-injection severely affected key modulators of muscle metabolism and histology. These data provide useful information for the development of strategies that aim to improve muscle molecular signaling and thereby recovery.

Skeletal Muscle and the Effects of Ammonia Toxicity in Fish, Mammalian, and Avian Species: A Comparative Review Based on Molecular Research

Typically, mammalian and avian models have been used to examine the effects of ammonia on skeletal muscle. Hyperammonemia causes sarcopenia or muscle wasting, in mammals and has been linked to sarcopenia in liver disease patients. Avian models of skeletal muscle have responded positively to hyperammonemia, differing from the mammalian response. Fish skeletal muscle has not been examined as extensively as mammalian and avian muscle. Fish skeletal muscle shares similarities with avian and mammalian muscle but has notable differences in growth, fiber distribution, and response to the environment. The wide array of body sizes and locomotion needs of fish also leads to greater diversity in muscle fiber distribution and growth between different fish species. The response of fish muscle to high levels of ammonia is important for aquaculture and quality food production but has not been extensively studied to date. Understanding the differences between fish, mammalian and avian species’ myogenic response to hyperammonemia could lead to new therapies for muscle wasting due to a greater understanding of the mechanisms behind skeletal muscle regulation and how ammonia effects these mechanisms. This paper provides an overview of fish skeletal muscle and ammonia excretion and toxicity in fish, as well as a comparison to avian and mammalian species.

5F peptide promotes endothelial differentiation of bone marrow stem cells through activation of ERK1/2 signaling

Synthetic apolipoprotein A-I (apoA-I) mimetic peptide 5F exhibits anti-atherosclerotic ability with largely unknown mechanism(s). Bone marrow (BM)-derived endothelial progenitor cells (EPCs) play a critical role in vascular integrity and function. The objective of the present study was to evaluate the effect of 5F on endothelial differentiation of BM stem cells and related mechanisms. Murine BM multipotent adult progenitor cells (MAPCs) were induced to differentiate into endothelial cells in vitro with or without 5F. The expression of endothelial markers vWF, Flk-1 and CD31 was significantly increased in the cells treated with 5F with enhanced in vitro vascular tube formation and LDL uptake without significant changes on proliferation and stem cell maker Oct-4 expression. Phosphorylated ERK1/2, not Akt, was significantly increased in 5F-treated cells. Treatment of MAPCs with PD98059 or small interfering RNA against ERK2 substantially attenuated ERK1/2 phosphorylation, and effectively prevented 5F-induced enhancement of endothelial differentiation of MAPCs. In vivo studies revealed that 5F increased EPCs number in the BM in mice after acute hindlimb ischemia that was effectively prevented with PD98059 treatment. These data supported the conclusion that 5F promoted endothelial differentiation of MAPCs through activation of ERK1/2 signaling.

Beef extract supplementation promotes myoblast proliferation and myotube growth in C2C12 cells

PURPOSE

We previously determined that the intake of beef extract for 4 weeks increases skeletal muscle mass in rats. Thus, this study aimed to clarify whether beef extract has a hypertrophic effect on muscle cells and to determine the signaling pathway underlying beef extract-induced myotube hypertrophy.

METHODS

We assessed the effects of beef extract supplement on mouse C2C12 skeletal muscle cell proliferation and differentiation and myotube growth. In addition, the phosphorylation of Akt, ERK1/2, and mTOR following beef extract supplementation was examined by western blotting. Furthermore, the bioactive constituents of beef extract were examined using amino acid analysis and dialysis.

RESULTS

In the proliferative stage, beef extract significantly increased myoblast proliferation. In the differentiation stage, beef extract supplementation did not promote myoblast differentiation. In mature myotubes, beef extract supplementation increased myotube diameter and promoted protein synthesis. Although Akt and ERK1/2 levels were not affected, beef extract supplementation increased mTOR phosphorylation, which indicated that the mTOR pathway mediates beef extract-induced myotube hypertrophy. The hypertrophic activity was observed in fractions of > 7000 Da.

CONCLUSIONS

Beef extract promoted C2C12 myoblast proliferation and C2C12 myotube hypertrophy. Myotube hypertrophy was potentially induced by mTOR activation and active components in beef extract were estimated to be > 7000 Da.

Chromatin accessibility is associated with the changed expression of miRNAs that target members of the Hippo pathway during myoblast differentiation

miRNAs reportedly participate in various biological processes, such as skeletal muscle proliferation and differentiation. However, the regulation of differentially expressed (DE) miRNAs and their function in myogenesis remain unclear. Herein, miRNA expression profiles and regulation during C2C12 differentiation were analyzed in relation to chromatin states by RNA-seq, ATAC-seq, and ChIP-seq. We identified 19 known and nine novel differentially expressed miRNAs at days 0, 1, 2, and 4. The expression of the differentially expressed miRNAs was related to the chromatin states of the 113 surrounding open chromatin regions defined by ATAC-seq peaks. Of these open chromatin regions, 44.25% were colocalized with MyoD/MyoG binding sites. The remainder of the above open chromatin regions were enriched with motifs of the myoblast-expressed AP-1 family, Ctcf, and Bach2 transcription factors (TFs). Additionally, the target genes of the above differentially expressed miRNAs were enriched primarily in muscle growth and development pathways, especially the Hippo signaling pathway. Moreover, via combining a loss-of-function assay with Q-PCR, western blotting, and immunofluorescence, we confirmed that the Hippo signaling pathway was responsible for C2C12 myoblast differentiation. Thus, our results showed that these differentially expressed miRNAs were regulated by chromatin states and affected muscle differentiation through the Hippo signaling pathway. Our findings provide new insights into the function of these differentially expressed miRNAs and the regulation of their expression during myoblast differentiation.

Modulation of osteogenic and myogenic differentiation by a phytoestrogen formononetin via p38MAPK-dependent JAK-STAT and Smad-1/5/8 signaling pathways in mouse myogenic progenitor cells

Formononetin (FN), a typical phytoestrogen has attracted substantial attention as a novel agent because of its diverse biological activities including, osteogenic differentiation. However, the molecular mechanisms underlying osteogenic and myogenic differentiation by FN in C2C12 progenitor cells remain unknown. Therefore the objective of the current study was to investigate the action of FN on myogenic and osteogenic differentiation and its impact on signaling pathways in C2C12 cells. FN significantly increased myogenic markers such as Myogenin, myosin heavy chains, and myogenic differentiation 1 (MyoD). In addition, the expression of osteogenic specific genes alkaline phosphatase (ALP), Run-related transcription factor 2(RUNX2), and osteocalcin (OCN) were up-regulated by FN treatment. Moreover, FN enhanced the ALP level, calcium deposition and the expression of bone morphogenetic protein isoform (BMPs). Signal transduction pathways mediated by p38 mitogen-activated protein kinase (p38MAPK), extracellular signal-related kinases (ERKs), protein kinase B (Akt), Janus kinases (JAKs), and signal transducer activator of transcription proteins (STATs) in myogenic and osteogenic differentiation after FN treatment were also examined. FN treatment activates myogenic differentiation by increasing p38MAPK and decreasing JAK1-STAT1 phosphorylation levels, while osteogenic induction was enhanced by p38MAPK dependent Smad, 1/5/8 signaling pathways in C2C12 progenitor cells.

Phosphorylated ERK1/2 protein levels are closely associated with the fast fiber phenotypes in rat hindlimb skeletal muscles

The relationship between the extracellular signal-regulated kinase 1 and 2 (ERK1/2), one of the mitogen-activated protein kinases (MAPKs), and mammalian skeletal muscle fiber phenotype is unclear. We looked at this relationship in three in vivo conditions in male Wistar rats. First, the levels of phosphorylated (active) ERK1/2 protein were closely associated with the fiber type composition of sedentary rat hindlimb muscles: highest in the superficial portion of the gastrocnemius (100% fast fibers), lower in the plantaris (~ 80% fast fibers), and lowest in the soleus (~ 15% fast fibers). Second, during growth, there was a gradual decrease in the percentage of fast fibers from 40% at 3 weeks to 1.5% at 65 weeks and a concomitant gradual decrease in the levels of phosphorylated ERK1/2 in the soleus muscle. Third, sciatic nerve denervation induced a significant decrease in the weight of both the soleus and plantaris, but a slow-to-fast fiber type shift and increase in phosphorylated ERK1/2 protein were observed only in the soleus. Although only a few fast and fast + slow hybrid fibers of the denervated soleus muscle reacted positively to the anti-phosphorylated ERK1/2 antibody by immuno-histochemical analysis, our results suggest that the phosphorylated form of ERK1/2 seems to be closely related to the fast fiber phenotype program. Further evidence for this relationship was provided by the observation that several slow fiber phenotype-specific proteins, i.e., Hsp72, Hsp60, and PGC-1, changed in the opposite direction of the levels of phosphorylated ERK1/2 protein.

Lactate Stimulates a Potential for Hypertrophy and Regeneration of Mouse Skeletal Muscle

The effects of lactate on muscle mass and regeneration were investigated using mouse skeletal muscle tissue and cultured C2C12 cells. Male C57BL/6J mice were randomly divided into (1) control, (2) lactate (1 mol/L in distilled water, 8.9 mL/g body weight)-administered, (3) cardio toxin (CTX)-injected (CX), and (4) lactate-administered after CTX-injection (LX) groups. CTX was injected into right tibialis anterior (TA) muscle before the oral administration of sodium lactate (five days/week for two weeks) to the mice. Oral lactate administration increased the muscle weight and fiber cross-sectional area, and the population of Pax7-positive nuclei in mouse TA skeletal muscle. Oral administration of lactate also facilitated the recovery process of CTX-associated injured mouse TA muscle mass accompanied with a transient increase in the population of Pax7-positive nuclei. Mouse myoblast-derived C2C12 cells were differentiated for five days to form myotubes with or without lactate administration. C2C12 myotube formation with an increase in protein content, fiber diameter, length, and myo-nuclei was stimulated by lactate. These observations suggest that lactate may be a potential molecule to stimulate muscle hypertrophy and regeneration of mouse skeletal muscle via the activation of muscle satellite cells.

Differential ammonia metabolism and toxicity between avian and mammalian species, and effect of ammonia on skeletal muscle: A comparative review

Comparative aspects of ammonia toxicity, specific to liver and skeletal muscle and skeletal muscle metabolism between avian and mammalian species are discussed in the context of models for liver disease and subsequent skeletal muscle wasting. The purpose of this review is to present species differences in ammonia metabolism and to specifically highlight observed differences in skeletal muscle response to excess ammonia in avian species. Ammonia, which is produced during protein catabolism and is an essential component of nucleic acid and protein biosynthesis, is detoxified mainly in the liver. While the liver is consistent as the main organ responsible for ammonia detoxification, there are evolutionary differences in ammonia metabolism and nitrogen excretory products between avian and mammalian species. In patients with liver disease and all mammalian models, inadequate ammonia detoxification and successive increased circulating ammonia concentration, termed hyperammonemia, leads to severe skeletal muscle atrophy, increased apoptosis and reduced protein synthesis, altogether having deleterious effects on muscle size and strength. Previously, an avian embryonic model, designed to determine the effects of increased circulating ammonia on muscle development, revealed that ammonia elicits a positive myogenic response. Specifically, induced hyperammonemia in avian embryos resulted in a reduction in myostatin, a well-known inhibitor of muscle growth, expression, whereas myostatin expression is significantly increased in mammalian models of hyperammonemia. These interesting findings imply that species differences in ammonia metabolism allow avians to utilize ammonia for growth. Understanding the intrinsic physiological mechanisms that allow for ammonia to be utilized for growth has potential to reveal novel approaches to muscle growth in avian species and will provide new targets for preventing muscle degeneration in mammalian species.

A promotive effect for halofuginone on membrane repair and synaptotagmin-7 levels in muscle cells of dysferlin-null mice

In the absence of dysferlin, skeletal muscle cells fail to reseal properly after injury, resulting in slow progress of the dysferlinopathy muscular dystrophy (MD). Halofuginone, a leading agent in preventing fibrosis in MDs, was tested for its effects on membrane resealing post-injury. A hypo-osmotic shock assay on myotubes derived from wild-type (Wt) and dysferlin-null (dysf-/-) mice revealed that pre-treatment with halofuginone reduces the percentage of membrane-ruptured myotubes only in dysf-/- myotubes. In laser-induced injury of isolated myofibers, halofuginone decreased the amount of FM1-43 at the injury site of dysf-/- myofibers while having no effect on Wt myofibers. These results implicate halofuginone in ameliorating muscle-cell membrane integrity in dysf-/- mice. Halofuginone increased lysosome scattering across the cytosol of dysf-/- primary myoblasts, in a protein kinase/extracellular signal-regulated protein kinase and phosphoinositide 3 kinase/Akt-dependent manner, in agreement with an elevation in lysosomal exocytotic activity in these cells. A spatial- and age-dependent synaptotagmin-7 (Syt-7) expression pattern was shown in dysf-/- versus Wt mice, suggesting that these pattern alterations are related to the disease progress and that sytnaptotagmin-7 may be compensating for the lack of dysferlin at least with regard to membrane resealing post-injury. While halofuginone did not affect patch-repair-complex key proteins, it further enhanced Syt-7 levels and its spread across the cytosol in dysf-/- myofibers and muscle tissue, and increased its co-localization with lysosomes. Together, the data imply a novel role for halofuginone in membrane-resealing events with Syt-7 possibly taking part in these events.

EPA and DHA Inhibit Myogenesis and Downregulate the Expression of Muscle-related Genes in C2C12 Myoblasts

This study was conducted to elucidate the biological effects of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) on cell proliferation, differentiation and gene expression in C2C12 myoblasts. C2C12 were treated with various concentrations of EPA or DHA under proliferation and differentiation conditions. Cell viability was analyzed using cell counting kit-8 assays (CCK-8). The Edu assays were performed to analyze cell proliferation. To analyze cell differentiation, the expressions of myogenic marker genes were determined at the transcriptional and translational levels by qRT-PCR, immunoblotting and immunofluorescence. Global gene expression patterns were characterized using RNA-sequencing. Phosphorylation levels of ERK and Akt were examined by immunoblotting. Cell viability and proliferation was significantly inhibited after incubation with EPA (50 and 100 μM) or DHA (100 μM). Both EPA and DHA suppressed C2C12 myoblasts differentiation. RNA-sequencing analysis revealed that some muscle-related genes were significantly downregulated following EPA or DHA (50 μM) treatment, including insulin-like growth factor 2 (IGF-2), troponin T3 (Tnnt3), myoglobin (Mb), myosin light chain phosphorylatable fast skeletal muscle (Mylpf) and myosin heavy polypeptide 3 (Myh3). IGF-2 was crucial for the growth and differentiation of skeletal muscle and could activate the PI3K/Akt and the MAPK/ERK cascade. We found that EPA and DHA (50 μM) decreased the phosphorylation levels of ERK1/2 and Akt in C2C12 myoblasts. Thus, this study suggested that EPA and DHA exerted an inhibitory effect on myoblast proliferation and differentiation and downregulated muscle-related genes expression.

MENS-associated increase of muscular protein content via modulation of caveolin-3 and TRIM72

Microcurrent electrical neuromuscular stimulation (MENS) is known as an extracellular stimulus for the regeneration of injured skeletal muscle in sports medicine. However, the effects of MENS-associated increase in muscle protein content are not fully clarified. The purpose of this study was to investigate the effects of MENS on the muscular protein content, intracellular signals, and the expression level of caveolin-3 (Cav-3), tripartite motif-containing 72 (TRIM72) and MM isoenzyme of creatine kinase (CK-MM) in skeletal muscle using cell culture system. C2C12 myotubes on the 7th day of differentiation phase were treated with MENS (intensity: 10-20 microA, frequency: 0.3 Hz, pulse width: 250 ms, stimulation time: 15-120 min). MENS-associated increase in the protein content of myotubes was observed, compared to the untreated control level. MENS upregulated the expression of Cav-3, TRIM72, and CK-MM in myotubes. A transient increase in phosphorylation level of Akt was also observed. However, MENS had no effect on the phosphorylation level of p42/44 extracellular signal-regulated kinase-1/2 and 5'AMP-activated protein kinase. MENS may increase muscle protein content accompanied with a transient activation of Akt and the upregulation of Cav-3 and TRIM72.

Platelet-rich plasma and alignment enhance myogenin via ERK mitogen activated protein kinase signaling

Volumetric muscle loss is debilitating and involves extensive rehabilitation. One approach to accelerate healing, rehabilitation, and muscle function is to repair damaged skeletal muscle using regenerative medicine strategies. In sports medicine and orthopedics, a common clinical approach is to treat minor to severe musculoskeletal injuries with platelet-rich plasma (PRP) injections. While these types of treatments have become commonplace, there are limited data demonstrating their effectiveness. The goal of this study was to determine the effect of PRP on myoblast gene expression and protein production when incorporated into a polymer fiber. To test this, we generated extracellular matrix mimicking scaffolds using aligned polydioxanone (PDO) fibers containing lyophilized PRP (SmartPReP^® 2, Harvest Technologies Corporation, Plymouth, MA). Scaffolds with PRP caused a dose-dependent increase in myogenin and myosin heavy chain but did not affect myogenic differentiation factor-1 (MyoD). Integrin α7β1D decreased and α5β1A did not change in response to PRP scaffolds. ERK inhibition decreased myogenin and increased Myod on the PDO-PRP scaffolds. Taken together, these data suggest that alignment and PRP produce a substrate-dependent, ERK-dependent, and dose-dependent effect on myogenic differentiation.

MEK inhibition induces MYOG and remodels super-enhancers in RAS-driven rhabdomyosarcoma

The RAS isoforms are frequently mutated in many types of human cancers, including PAX3/PAX7 fusion-negative rhabdomyosarcoma. Pediatric RMS arises from skeletal muscle progenitor cells that have failed to differentiate normally. The role of mutant RAS in this differentiation blockade is incompletely understood. We demonstrate that oncogenic RAS, acting through the RAF-MEK [mitogen-activated protein kinase (MAPK) kinase]-ERK (extracellular signal-regulated kinase) MAPK effector pathway, inhibits myogenic differentiation in rhabdomyosarcoma by repressing the expression of the prodifferentiation myogenic transcription factor, MYOG. This repression is mediated by ERK2-dependent promoter-proximal stalling of RNA polymerase II at the MYOG locus. Small-molecule screening with a library of mechanistically defined inhibitors showed that RAS-driven RMS is vulnerable to MEK inhibition. MEK inhibition with trametinib leads to the loss of ERK2 at the MYOG promoter and releases the transcriptional stalling of MYOG expression. MYOG subsequently opens chromatin and establishes super-enhancers at genes required for late myogenic differentiation. Furthermore, trametinib, in combination with an inhibitor of IGF1R, potently decreases rhabdomyosarcoma cell viability and slows tumor growth in xenograft models. Therefore, this combination represents a potential therapeutic for RAS-mutated rhabdomyosarcoma.

Lactate increases myotube diameter via activation of MEK/ERK pathway in C2C12 cells

AIM

Lactate is produced in and released from skeletal muscle cells. Lactate receptor, G-protein-coupled receptor 81 (GPR81), is expressed in skeletal muscle cells. However, a physiological role of extracellular lactate on skeletal muscle is not fully clarified. The purpose of this study was to investigate extracellular lactate-associated morphological changes and intracellular signals in C2C12 skeletal muscle cells.

METHODS

Mouse myoblast C2C12 cells were differentiated for 5 days to form myotubes. Sodium lactate (lactate) or GPR81 agonist, 3,5-dihydroxybenzoic acid (3,5-DHBA), was administered to the differentiation medium.

RESULTS

Lactate administration increased the diameter of C2C12 myotubes in a dose-dependent manner. Administration of 3,5-DHBA also increased myotube diameter. Not only lactate but also 3,5-DHBA upregulated the phosphorylation level of mitogen-activated protein kinase kinase 1/2 (MEK1/2), p42/44 extracellular signal-regulated kinase-1/2 (ERK1/2) and p90 ribosomal S6 kinase (p90RSK). MEK inhibitor U0126 depressed the phosphorylation of ERK-p90RSK and increase in myotube diameter induced by lactate. On the other hand, both lactate and 3,5-DHBA failed to induce significant responses in the phosphorylation level of Akt, mammalian target of rapamycin, p70 S6 kinase and protein degradation-related signals.

CONCLUSION

These observations suggest that lactate-associated increase in the diameter of C2C12 myotubes is induced via activation of GRP81-mediated MEK/ERK pathway. Extracellular lactate might have a positive effect on skeletal muscle size.

Potential Roles of n-3 PUFAs during Skeletal Muscle Growth and Regeneration

Omega-3 polyunsaturated fatty acids (n-3 PUFAs), which are commonly found in fish oil supplements, are known to possess anti-inflammatory properties and more recently alter skeletal muscle function. In this review, we discuss novel findings related to how n-3 PUFAs modulate molecular signaling responsible for growth and hypertrophy as well as the activity of muscle stem cells. Muscle stem cells commonly known as satellite cells, are primarily responsible for driving the skeletal muscle repair process to potentially damaging stimuli, such as mechanical stress elicited by exercise contraction. To date, there is a paucity of human investigations related to the effects of n-3 PUFAs on satellite cell content and activity. Based on current in vitro investigations, this review focuses on novel mechanisms linking n-3 PUFA’s to satellite cell activity and how they may improve muscle repair. Understanding the role of n-3 PUFAs during muscle growth and regeneration in association with exercise could lead to the development of novel supplementation strategies that increase muscle mass and strength, therefore possibly reducing the burden of muscle wasting with age.

Ranolazine promotes muscle differentiation and reduces oxidative stress in C2C12 skeletal muscle cells

PURPOSE

The purpose of this study is to investigate Ranolazine action on skeletal muscle differentiation and mitochondrial oxidative phenomena. Ranolazine, an antianginal drug, which acts blocking the late INaL current, was shown to lower hemoglobin A1c in patients with diabetes. In the present study, we hypothesized an action of Ranolazine on skeletal muscle cells regeneration and oxidative process, leading to a reduction of insulin resistance.

METHODS

10 μM Ranolazine was added to C2C12 murine myoblastic cells during proliferation, differentiation and newly formed myotubes.

RESULTS

Ranolazine promoted the development of a specific myogenic phenotype: increasing the expression of myogenic regulator factors and inhibiting cell cycle progression factor (p21). Ranolazine stimulated calcium signaling (calmodulin-dependent kinases) and reduced reactive oxygen species levels. Furthermore, Ranolazine maintained mitochondrial homeostasis. During the differentiation phase, Ranolazine promoted myotubes formation. Ranolazine did not modify kinases involved in skeletal muscle differentiation and glucose uptake (extracellular signal-regulated kinases 1/2 and AKT pathways), but activated calcium signaling pathways. During proliferation, Ranolazine did not modify the number of mitochondria while decreasing osteopontin protein levels. Lastly, neo-formed myotubes treated with Ranolazine showed typical hypertrophic phenotype.

CONCLUSION

In conclusion, our results indicate that Ranolazine stimulates myogenesis and reduces a pro-oxidant inflammation/oxidative condition, activating a calcium signaling pathway. These newly described mechanisms may partially explain the glucose lowering effect of the drug.

Copy number variants of Ras/MAPK pathway genes in patients with isolated cryptorchidism

Cryptorchidism is the most common congenital disorder in boys, but the cause for most cases remains unknown. Patients with Noonan Syndrome are characterized by a typical face, growth retardation, congenital heart defects, learning disabilities and cryptorchidism. Copy number variations of Ras/MAPK pathway genes are unusual in patients with several clinical features of Noonan Syndrome; however, they have not been studied in patients with only one feature of this condition, such as cryptorchidism. Our aim was to determine whether patients with isolated cryptorchidism exhibit Ras/MAPK pathway gene copy number variations (CNVs). Fifty-nine patients with isolated cryptorchidism and negative for mutations in genes associated with Noonan Syndrome were recruited. Determination of Ras/MAPK pathway gene CNVs was performed by Comparative Genome Hybridization array. A CNV was identified in two individuals, a ~175 kb microduplication at 3p25.2, partially including RAF1. A similar RAF1 microduplication has been observed in a patient with testicular aplasia. This suggests that some patients with isolated cryptorchidism may harbor Ras/MAPK pathway gene CNVs.

Mice overexpressing growth hormone exhibit increased skeletal muscle myostatin and MuRF1 with attenuation of muscle mass

Background

In contrast to the acute effects of growth hormone (GH) on skeletal muscle protein synthesis, long-term GH treatment appears to have negligible effects on muscle mass. Despite this knowledge, little is known regarding the chronic effects of GH on skeletal muscle protein synthesis and atrophy signaling pathways. The purpose of this study was to determine if protein synthesis pathways are attenuated and/or muscle atrophy intracellular signaling pathways are altered in the skeletal muscle of transgenic bovine GH (bGH) mice.

Methods

The gastrocnemius and soleus from 5-month-old male bGH mice (n = 9) and wild type (WT) controls (n = 9) were harvested and analyzed for proteins involved in the protein synthesis (Akt/mTOR), growth and proliferation (MAPK), and muscle atrophy (MuRF1 and myostatin) pathways.

Results

Total body mass was significantly increased in bGH mice compared to WT controls (49%, P < 0.0001). When expressed relative to total body mass, the gastrocnemius (− 28%, P < 0.0001), but not the soleus, was significantly lower in mice overexpressing GH, compared to controls. Transgenic bGH mice had elevated phosphorylation levels of protein kinase b (Akt1), 4E-binding protein 1 (4E-BP1), p70 S6 kinase, p42/44, and p38 (P < 0.05) compared to WT littermates. Mature myostatin (26 kDa), premature myostatin (52 kDa), and activin receptor type IIB (AcvR2B) protein levels were increased in bGH mice (P < 0.05), along with elevated phosphorylation levels of mothers against decapentaplegic homolog (Smad2) (59%, P < 0.0001). Mice overexpressing GH had increased MuRF1 expression (30%, P < 0.05) and insulin receptor substrate 1 (IRS1) serine phosphorylation (44%, P < 0.05) in the gastrocnemius, but not the soleus, when compared to controls.

Conclusions

These findings demonstrate that chronic elevations in circulating GH have a critical impact on signaling pathways involved in skeletal muscle protein synthesis and atrophy, and suggest that MuRF1, myostatin, and IRS1 serine phosphorylation may act to inhibit exaggerated glycolytic muscle growth, in environments of chronic GH/IGF-1 excess.

ERK2 and JNK1 contribute to TNF-α-induced IL-8 expression in synovial fibroblasts

Tumor necrosis factor α (TNF-α) induces the expression and secretion of interleukin 8 (IL-8), which contributes to synovitis in rheumatoid arthritis (RA). To elucidate the mechanism of the onset of RA, we used synovial fibroblasts without autoimmune inflammatory diseases and investigated MAPK signaling pathways in TNF-α-induced IL-8 expression. Synovial fibroblasts isolated from healthy dogs were characterized by flow cytometry, which were positive for the fibroblast markers CD29, CD44, and CD90 but negative for the hematopoietic cell markers CD14, CD34, CD45, and HLA-DR. TNF-α stimulated the secretion and mRNA expression of IL-8 in a time- and dose-dependent manner. ERK and JNK inhibitors attenuated TNF-α-induced IL-8 expression and secretion. TNF-α induced the phosphorylation of ERK1/2 and JNK1/2. TNF-α-induced IL-8 expression was attenuated both in ERK2- and JNK1-knockdown cells. TNF-α-induced ERK1/2 or JNK1/2 was observed in ERK2- or JNK1-knockdown cells, respectively, showing that there is no crosstalk between ERK2 and JNK1 pathways. These observations indicate that the individual activation of ERK2 and JNK1 pathways contributes to TNF-α-induced IL-8 expression in synovial fibroblasts, which appears to be involved in the progress in RA.

Adipose stem cells enhance myoblast proliferation via acetylcholine and extracellular signal-regulated kinase 1/2 signaling

Introduction: In this study we investigated the interaction between adipose tissue–derived stem cells (ASCs) and myoblasts in co‐culture experiments. Methods: Specific inductive media were used to differentiate ASCs in vitro into a Schwann cell–like phenotype (differentiated adipose tissue–derived stem cells, or dASCs) and, subsequently, the expression of acetylcholine (ACh)‐related machinery was determined. In addition, the expression of muscarinic ACh receptors was examined in denervated rat gastrocnemius muscles. Results: In contrast to undifferentiated ASCs, dASCs expressed more choline acetyltransferase and vesicular acetylcholine transporter. When co‐cultured with myoblasts, dASCs enhanced the proliferation rate, as did ACh administration alone. Western blotting and pharmacological inhibitor studies showed that phosphorylated extracellular signal–regulated kinase 1/2 signaling mediated these effects. In addition, denervated muscle showed higher expression of muscarinic ACh receptors than control muscle. Discussion: Our findings suggest that dASCs promote proliferation of myoblasts through paracrine secretion of ACh, which could explain some of their regenerative capacity in vivo. Muscle Nerve57: 305–311, 2018

Akirin2 regulates proliferation and differentiation of porcine skeletal muscle satellite cells via ERK1/2 and NFATc1 signaling pathways

Akirin2, a novel nuclear factor, plays an important role in myogenesis. To investigate the role of Akirin2 in proliferation and differentiation of porcine skeletal muscle satellite cells, Akirin2 overexpression and Akirin2 silence technologies were employed. Our results showed that overexpression of Akirin2 markedly enhanced the proliferation and differentiation of porcine skeletal muscle satellite cells, whereas silencing of Akirin2 got the opposite results. Furthermore, our results showed that Akirin2 affected proliferation and differentiation of porcine skeletal muscle satellite cells through extracellular-signal regulated kinase-1/2 (ERK1/2) and NFATc1 signaling pathways. These results indicate that Akirin2 can effectively promote skeletal muscle satellite cells proliferation and differentiation, acting through ERK1/2- and NFATc1-dependent mechanisms.

14-3-3β Depletion Drives a Senescence Program in Glioblastoma Cells Through the ERK/SKP2/p27 Pathway

The induction of senescence in cancer cells has recently been implicated as a mechanism of tumor regression in response to various modes of stress. 14-3-3 proteins are conserved scaffolding molecules that are involved in various cellular functions. Among the seven isoforms, 14-3-3β is specifically expressed in astrocytoma in correlation with the malignancy grade. We investigated the possible role of 14-3-3β in the regulation of senescence induction in A172 glioblastoma cells. The knockdown of 14-3-3β by specific small interfering RNA resulted in a significant change in cellular phenotypes and an increase in cells staining positive for senescence-associated β-galactosidase. Western blotting of the 14-3-3β-depleted A172 cells revealed increased p27 expression and decreased SKP2 expression, while the expression of p53 and p21 was not altered. Subsequently, we demonstrated that ERK is a key modulator of SKP2/p27 axis activity in 14-3-3β-mediated senescence based on the following: (1) 14-3-3β knockdown decreased p-ERK levels; (2) treatment with U0126, an MEK inhibitor, completely reproduced the senescence morphology as well as the expression profiles of p27 and SKP2; and (3) the senescence phenotypes induced by 14-3-3β depletion were considerably recovered by constitutively active ERK expression. Our results indicate that 14-3-3β negatively regulates senescence in glioblastoma cells via the ERK/SKP2/p27 pathway. Furthermore, 14-3-3β depletion also resulted in senescence phenotypes in U87 glioblastoma cells, suggesting that 14-3-3β could be targeted to induce premature senescence as a therapeutic strategy against glioblastoma progression.

The MEK-Inhibitor Selumetinib Attenuates Tumor Growth and Reduces IL-6 Expression but Does Not Protect against Muscle Wasting in Lewis Lung Cancer Cachexia

Cachexia, or wasting of skeletal muscle and fat, afflicts many patients with chronic diseases including cancer, organ failure, and AIDS. Muscle wasting reduces quality of life and decreases response to therapy. Cachexia is caused partly by elevated inflammatory cytokines, including interleukin-6 (IL-6). Others and we have shown that IL-6 alone is sufficient to induce cachexia both in vitro and in vivo. The mitogen-activated protein/extracellular signal-regulated kinase kinase (MEK) inhibitor Selumetinib has been tested in clinical trials for various cancers. Moreover, Selumetinib has also been shown to inhibit the production of IL-6. In a retrospective analysis of a phase II clinical trial in advanced cholangiocarcinoma, patients treated with Selumetinib experienced significant gains in skeletal muscle vs. patients receiving standard therapy. However, the use of Selumetinib as a treatment for cachexia has yet to be investigated mechanistically. We sought to determine whether MEK inhibition could protect against cancer-induced cachexia in mice. In vitro, Selumetinib induced C2C12 myotube hypertrophy and nuclear accretion. Next we tested Selumetinib in the Lewis lung carcinoma (LLC) model of cancer cachexia. Treatment with Selumetinib reduced tumor mass and reduced circulating and tumor IL-6; however MEK inhibition did not preserve muscle mass. Similar wasting was seen in limb muscles of Selumetinib and vehicle-treated LLC mice, while greater fat and carcass weight loss was observed with Selumetinib treatment. As well, Selumetinib did not block wasting in C2C12 myotubes treated with LLC serum. Taken together, out results suggest that this MEK inhibitor is not protective in LLC cancer cachexia despite lowering IL-6 levels, and further that it might exacerbate tumor-induced weight loss. Differences from other studies might be disease, species or model-specific.

Controlled Heat Stress Promotes Myofibrillogenesis during Myogenesis

Hyperthermia therapy has recently emerged as a clinical modality used to finely tune heat stress inside the human body for various biomedical applications. Nevertheless, little is known regarding the optimal timing or temperature of heat stress that is needed to achieve favorable results following hyperthermia therapy for muscle regeneration purposes. The regeneration of skeletal muscle after injury is a highly complex and coordinated process that involves a multitude of cellular mechanisms. The main objective of this study was to characterize the effects of hyperthermal therapy on the overall behavior of myoblasts during myogenic differentiation. Various cellular processes, including myogenesis, myofibrillogenesis, hypertrophy/atrophy, and mitochondrial biogenesis, were studied using systematic cellular, morphological, and pathway-focused high-throughput gene expression profiling analyses. We found that C2C12 myoblasts exhibited distinctive time and temperature-dependence in biosynthesis and regulatory events during myogenic differentiation. Specifically, we for the first time observed that moderate hyperthermia at 39°C favored the growth of sarcomere in myofibrils at the late stage of myogenesis, showing universal up-regulation of characteristic myofibril proteins. Characteristic myofibrillogenesis genes, including heavy polypeptide 1 myosin, heavy polypeptide 2 myosin, alpha 1 actin, nebulin and titin, were all significantly upregulated (p<0.01) after C2C12 cells differentiated at 39°C over 5 days compared with the control cells cultured at 37°C. Furthermore, moderate hyperthermia enhanced myogenic differentiation, with nucleus densities per myotube showing 2.2-fold, 1.9-fold and 1.6-fold increases when C2C12 cells underwent myogenic differentiation at 39°C over 24 hours, 48 hours and 72 hours, respectively, as compared to the myotubes that were not exposed to heat stress. Yet, atrophy genes were sensitive even to moderate hyperthermia, indicating that strictly controlled heat stress is required to minimize the development of atrophy in myotubes. In addition, mitochondrial biogenesis was enhanced following thermal induction of myoblasts, suggesting a subsequent shift toward anabolic demand requirements for energy production. This study offers a new perspective to understand and utilize the time and temperature-sensitive effects of hyperthermal therapy on muscle regeneration.

Lrrc75b is a novel negative regulator of C2C12 myogenic differentiation

Many transcription factors and signaling molecules involved in the guidance of myogenic differentiation have been investigated in previous studies. However, the precise molecular mechanisms of myogenic differentiation remain largely unknown. In the present study, by performing a meta-analysis of C2C12 myogenic differentiation microarray data, we found that leucine-rich repeat-containing 75B (Lrrc75b), also known as AI646023, a molecule of unknown biological function, was downregulated during C2C12 myogenic differentiation. The knockdown of Lrrc75b using specific siRNA in C2C12 myoblasts markedly enhanced the expression of muscle-specific myogenin and increased myoblast fusion and the myotube diameter. By contrast, the adenovirus-mediated overexpression of Lrrc75b in C2C12 cells markedly inhibited myoblast differentiation accompanied by a decrease in myogenin expression. In addition, the phosphorylation of extracellular signal-regulated kinase 1/2 (Erk1/2) was suppressed in the cells in which Lrrc75b was silenced. Taken together, our results demonstrate that Lrrc75b is a novel suppressor of C2C12 myogenic differentiation by modulating myogenin and Erk1/2 signaling.